A filter is a filter its physical orientation probably has no effect on how it operates other than it needs to be between control valve and tank... Use what ever plumbing that is appropriate to locate it in safe area... I would bet a couple of nipples and a 90° elbow would do...
As for changing hydraulic fluid, my splitter is over 30 years old, don't know if previous owners ever change fluids so I have no idea of how many hours on it.... But it uses Dexron II (ATF) and if a car can go 100,000 miles without auto trans fluid change, I seriously doubt the need to change fluid every 100 hours...
Dale